They're valid currency. I thinks that's what the sites and sellers documenting them mean by "uncirculated" as in the condition is not circulated and they have not been issued to banks.

Anyway Im pretty sure it was accepted as tender.. Maybe they talked me out if it. They asked me if I was sure. Can't really remember. anyone could have just swapped it for a fiver and probably did.

Edit: it's legal tender but that only actually means that it can be used to settle a debt in court. Shops aren't even obliged to accept change or coins over a certain amount of denominations. But they can choose to accept a £5 or £20 coin, although they would probably have to go through the hassle of selling it to someone else to get more widely accepted tender back, as even banks won't take them in (they don't have the machinery to weigh or count it).